On Jan 29, 6:22 pm, iMath <redstone-c...@163.com> wrote: > 在 2013年1月29日星期二UTC+8下午9时21分16秒，iMath写道： > > > why [os.path.join(r'E:\Python', name) for name in []] returns [] ? please > > explain it in detail ！ > >>> [os.path.join(r'E:\Python', name) for name in []] > > []
[Small algebra lesson] In algebra there is the concept of identity and absorbent. For example, 1 is the identity for multiply and 0 is the absorbent. ie for all x: 1 * x = x and 0 * x = 0 [end algebra lesson] In the case of lists, [] is an identity for ++ but behaves like an absorbent for comprehensions. Modern terminology for 'absorbent' is 'zero-element'. I personally find the older terminology more useful. Others have pointed out why operationally [] behaves like an absorbent in comprehensions. Ive seen even experienced programmers trip up on this so I believe its good to know it as an algebraic law in addition to the operational explanation. -- http://mail.python.org/mailman/listinfo/python-list
